Several proteins involved in in vitro DNA synthesis have been purified from E. coli. These proteins include those that are genetically identified and required for E. coli chromosome replication, namely, dnaB, dnaC(D), dnaE, dnaG, and dnaZ gene products, and those that are identified by their requirement in reconstituted DNA replication reactions, namely, DNA binding protein, replication factors X, Y, and Z, and DNA elongation factors I and III (DNA EF I and III). DNA synthesis reactions catalyzed by these proteins have been dissected into partial reactions. DNA synthesis with single-stranded phage DNA first requires the synthesis of an oligonucleotide primer for DNA elongation. With fd DNA, this is accomplished by RNA polymerase. With G4 and ST-1 DNA, dnaG protein catalyzes the synthesis of specific oligonucleotides using ADP and dNTPs or rNTPs. With phi X174 DNA, reactions involving dnaB and dnaC(D) proteins, DNA binding protein, and replication factors X, Y, and Z precede oligonucleotide synthesis by dnaG protein. DNA elongation of primer single-stranded DNA required dnaZ protein, DNA polymerase III, DNA EF I, and DNA EF III. Such interactions of replication proteins with each other, nucleotides, and DNA have been studied.